The present invention relates to an intrusion detection apparatus and a method by which the apparatus operates. More particularly, the present invention relates to an intrusion detection apparatus having a memory that simulates a capacitor and a method of operating the apparatus.
Intrusion detection apparatuses that utilize ultrasonic signals and the Doppler principle are known. Each of these apparatuses transmits a known frequency signal and monitors the frequency of a return signal. Movement within a protected area may cause a change in the frequency of the return signal as compared to the transmitted signal. The change in frequency of the signal is known as a Doppler shift.
One known intrusion detection apparatus applies the return signal, after filtering, to the circuit shown in FIG. 10. The circuit in FIG. 10 includes a window comparator comprising first and second op-amps 1002 and 1004, respectively. The first op-amp 1002 has a reference voltage of 2.7 volts applied to the inverting input and the filtered return signal applied to the non-inverting input. The second op-amp 1004 of the window comparator has a reference voltage of 2.3 volts applied to the non-inverting input and the filtered return signal applied to the inverting input.
An output 1006 of the window comparator is attached to a current source 1008. When the filtered return signal is within the range of 2.3 to 2.7 volts, the current source 1008 is not connected to a capacitor 1010 of the circuit. However, when the filtered return signal is outside of the range of 2.3 to 2.7 volts, the current source 1008 is attached to the capacitor 1010.
When the current source 1008 is attached to the capacitor 1010, the capacitor 1010 charges. When the capacitor 1010 is not connected to the current source 1008, leakage causes the capacitor 1010 to discharge. The capacitor 1010 is attached to the non-inverting input of a third comparator 1012. A 2.5 volt reference voltage is attached to the inverted input of the third comparator 1012. When the capacitor charge reaches 2.5 volts, the output 1014 of the third comparator 1012 is maximized and an alarm is triggered.
When the circuit of FIG. 10 is used in an intrusion detection apparatus, the apparatus is not adaptable to varying conditions. For example, the reference voltages applied to the first and second op-amps 1002 and 1004 fix the range of the comparator window. Since the reference voltages are not variable, changing environmental conditions are more likely to result in false alarms.
The present invention is an apparatus for detecting an intrusion into an area. The apparatus comprises a transmitter for transmitting a signal within the area. The apparatus also comprises a receiver for receiving reflected return signals of the transmitted signal and for generating an output signal indicative of the reflected return signals received. The apparatus also includes a controller for analyzing the output signal. The controller includes a processor for running an algorithm that simulates performance of a capacitor, which charges and discharges in response to an output signal, by increasing a virtual capacitor value when the output signal is greater than a threshold and by decreasing the virtual capacitor value when the output signal is less than the threshold.
In another aspect of the invention, a method for detecting an intrusion into an area is provided. The method comprises the steps of transmitting a signal within the area; receiving reflected return signals of the transmitted signal; generating an output signal indicative of the reflected return signals received; and running an algorithm that simulates performance of a capacitor, which charges and discharges in response to an output signal, by increasing a virtual capacitor value when the output signal is greater than a threshold and by decreasing the virtual capacitor value when the output signal is less than the threshold.